This is a long shot, so consider it just the ravings of an insomniac:
It is possible that the enclosure is a transmission line/tuned port hybrid of sorts, and if so, you might be able to tighten up the bottom end a bit by lowering the tuning frequency. One way to do so would be to reduce the cross-sectional area of the port over its entire length.
Remove the stuffing from the port, then measure how far the port extends into the box (horizontally). Cut several boards that long, and just wide enough to fit into the port. Stack enough such boards in the port to roughly cut its cross-sectional area in half.
If the box functions as a reflex box, we've just lowered the tuning frequency somewhat. And that just might be a better "fix" than stuffing the port with damping material. If the results seem promising, find the optimum number of boards by trial-and-error, and then if you want to, glue and/or screw the boards together to make a solid block, and maybe wrap the ends with electrical tape or some such to give you a snug, rattle-free friction fit.
Theaudiotweak's suggestion of improving the coupling between speaker and floor may work very well, I don't know, I have less experience with that approach. I don't see how it could do any harm (unlike the things I've recommended, which very well could be detrimental, especially in the wrong dosages... but at least they're easily reversible).
Duke
It is possible that the enclosure is a transmission line/tuned port hybrid of sorts, and if so, you might be able to tighten up the bottom end a bit by lowering the tuning frequency. One way to do so would be to reduce the cross-sectional area of the port over its entire length.
Remove the stuffing from the port, then measure how far the port extends into the box (horizontally). Cut several boards that long, and just wide enough to fit into the port. Stack enough such boards in the port to roughly cut its cross-sectional area in half.
If the box functions as a reflex box, we've just lowered the tuning frequency somewhat. And that just might be a better "fix" than stuffing the port with damping material. If the results seem promising, find the optimum number of boards by trial-and-error, and then if you want to, glue and/or screw the boards together to make a solid block, and maybe wrap the ends with electrical tape or some such to give you a snug, rattle-free friction fit.
Theaudiotweak's suggestion of improving the coupling between speaker and floor may work very well, I don't know, I have less experience with that approach. I don't see how it could do any harm (unlike the things I've recommended, which very well could be detrimental, especially in the wrong dosages... but at least they're easily reversible).
Duke